Re: [PATCH v2 0/5] Test overlayfs readdir cache

Miklos Szeredi
2021-04-26T10:07:57Z

On Sun, Apr 25, 2021 at 9:14 AM Amir Goldstein <amir73il@gmail.com> wrote:
>
> Eryu,
>
> This extends the generic t_dir_offset2 helper program to verify
> some cases of missing/stale entries and adds a new generic test which
> passes on overlayfs (and other fs) on upstream kernel.
>
> The overlayfs specific test fails on upstream kernel and the fix commit
> is currently in linux-next. As usual, you may want to wait with merging
> until the fix commit hits upstream.
>
> Based on feedback from Miklos, I changed the test to check for the
> missing/stale entries on a new fd, while old fd is kept open, because
> POSIX allows for stale/missing entries in the old fd.
>
> I was looking into another speculated bug in overlayfs which involves
> multiple calls to getdents. Although it turned out that overlayfs does
> not have the speculated bug, I left both generic and overlay test with
> multiple calls to getdents in order to excersize the relevant code.
>
> The attached patch was used to verify that the overlayfs test excercises
> the call to ovl_cache_update_ino() with stale entries.
> Overlayfs populates the merge dir readdir cache with a list of files in
> the first getdents call, but updates d_ino of files on the list in
> subsequent getdents calls. By that time, the last entry is stale and the
> following warning is printed (on linux-next with patch below applied):
> [ ] overlayfs: failed to look up (m100) for ino (0)
> [ ] overlayfs: failed to look up (f100) for ino (0)
>
> Miklos,
>
> Do you think it is worth the trouble to set p->is_whiteout and skip
> dir_emit() in this case? and do we need to worry about lookup_one_len()
> returning -ENOENT in this case?

So lookup_one_len() first does a cached lookup, and if found returns
that. If not then it calls the filesystem's ->lookup() callback,
which in this case is ovl_lookup(). AFAICS ovl_lookup() will never
return ENOENT, even if the underlying filesystem does.

Which means it's not necessary to worry about that case.

The other case you found it that in case of a stale direntry the i_ino
update will be skipped and so it will return an inconsistent result,
right? Fixing that looks worthwhile, yes.

Thanks,
Miklos